The owner of the Inland Revenue building in Cashel St has drawn up plans to rescue the formerly condemned building.

Building owner Simon Henry had originally decided to demolish the eight-storey office block, which was completed about five years ago, but now plans to repair the building and put it on seismic shock absorbers at a cost of $7 million to $15m.

But the building could still be bought by the Government and demolished.

It was placed in the green frame in the recovery blueprint unveiled by the Christchurch Earthquake Recovery Authority (Cera) in July.

Henry said the possibility of a Government offer for the building had not influenced his change of plans.

"This is not a pie-in-the-sky ratcheting mechanism," he said. "Cera are not into being hoodwinked. The building is what it is and the strengthening programme is what it is. It has been drawn up by well-qualified engineers."

Henry started to consider saving the building before the Government unveiled its recovery blueprint. He has not yet received an offer for the property.

Engineering firm Beca has drawn up plans to repair the steel frame and put the building on seismic shock absorbers - a system known as base isolation.

"If Cera don't buy the building, we will not demolish it. We will strengthen it and reopen it," Henry said.

"We have done a lot more engineering work on it and come up with a method to base-isolate the building, which is the holy grail of seismic strengthening."

Henry had spent "several hundred thousand dollars" on consultancy fees for the plan, which had been put on hold since the recovery blueprint was made public.

"I would be seeking consent by now if it wasn't for the Government's plans," he said.

"I can state categorically that if [the Government] decide not to buy it, I would move with great speed to fix it and open it."

If he does not like the Government's offer, he will contest it in the courts.

The building was completed in 2007. Henry has owned the site since 1993 and built the $50m building to replace one destroyed in a fire in 2005.